Half rate precoded data RZ transmitter

ABSTRACT

An optical duobinary transmitter. The transmitter uses a half-rate precoder, half-rate non-linear modulation drive circuits and a multiplex modulator for generating duobinary modulation on an optical signal from which full-rate data can be detected without decoding. The intensity of the optical signal is modulated to be zero between data symbols.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation-in-part of application Ser. No.10/299,425 filed Nov. 18, 2002 now U.S. Pat. No. 6,804,472. The presentinvention and the invention of application Ser. No. 10/299,425 wereowned by the same entity at the time the inventions were made.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to duobinary optical signalgeneration and more particularly to an optical transmitter and methodusing half rate data streams for generating full rate modulation in aduobinary optical signal.

2. Description of the Prior Art

Recently, optical duobinary techniques have attracted attention fornarrowing the spectrum of a transmitted optical signal and reducing thewaveform distortion that is induced by optical fiber chromaticdispersion. The spectrum of the transmitted signal is reduced by afactor of about two by mapping a binary data signal to be transmittedinto a three-level duobinary signal, with redundancy within the threelevels, to represent the binary data. While there are several techniquesfor implementing duobinary mapping onto an optical carrier, all of thetechniques result in the transmission of equivalent optical signals thattake on one of three possible optical electric-field amplitude values,with certain normalization, of {−1, 0, 1}.

The transmitters for generating these optical signals have electroniccircuits for generating signals for driving an optical modulator. Oneimportant limitation for these electronic circuits is data rate. Ingeneral, the higher the date rate, the more difficult it is to designthe circuits and the more expensive they are to manufacture. A secondlimitation is linearity. In general, it is less difficult and lessexpensive, and higher data rates are possible, when the electroniccircuits are not required to be linear.

The U.S. Pat. No. 5,867,534 by Price and Uhel; and papers “ReducedBandwidth Optical Digital Intensity Modulation with Improved ChromaticDispersion Tolerance” published in Electronics Letters, vol. 31, no. 1,in 1995 by A. J. Price and N. Le Mercier, and “210 km Repeaterless 10Gb/s Transmission Experiment through Nondispersion-Shifted Fiber UsingPartial Response Scheme” published in the IEEE Photonics TechnologyLetters in 1995 by A. J. Price, L. Pierre, R. Uhel and V. Havard reportthe usage of a low-pass filter to generate the three-level duobinarysignal and an optical duobinary technique where a redundancy is given tooptical phase. However, because the input of the low-pass filter is thefull-rate non-return-to-zero (NRZ) data, full-speed electronic circuitsare required.

The U.S. Pat. No. 5,543,952; and papers “Optical Duobinary TransmissionSystem with no Receiver Sensitivity Degradation” published in ElectronicLetters in 1995 by K. Yonenaga, S. Kuwano, S. Norimatsu and N. Shibata,and “Dispersion-Tolerant Optical Transmission System using DuobinaryTransmitter and Binary Receiver” published in the Journal of LightwaveTechnology in 1997 by K. Yonenaga and S. Kuwano report the usage of adelay-and-add circuit to generate the three-level duobinary signal andan optical duobinary technique where a redundancy is given to opticalphase. Again, because the input of the delay-and-add circuit is thefull-rate NRZ data, full-speed electronic circuits are required.

In both the U.S. Pat. Nos. 5,543,952 and 5,867,534, electronic modulatordrivers may operate at a bandwidth less than one-half the system datarate. However, the modulation drivers are required to be linear in orderto handle the three levels of the duobinary signal.

The U.S. Pat. Nos. 5,917,638 and 6,188,497 by Franck et al., and a paperby T. Franck, P. B. Hansen, T. N. Nielsen, and L. Eskildsen entitled“Duobinary Transmitter with Low Intersymbol Interference” published inIEEE Photonics Technology Letters in 1998 report a duobinary transmitterhaving dual binary modulation signals for driving a modulator. In asimplified view, an optical modulator is used as an adder for thedelay-and-add circuit used in the U.S. Pat. No. 5,543,952. However,full-rate circuits are again required as both modulation signals havethe same data rate as the optical signal.

The U.S. Pat. No. 6,337,756; and papers “A Dual-Drive Ti:LiNbO₃Mach-Zehnder Modulator Used as an Optoelectronic logic gate for 10-Gb/sSimultaneous Multiplexing and Modulation” published in IEEE PhotonicsTechnology Letters in 1992 of P. B. Hansen and A. H. Gnauck, and“Prechirped Duobinary Modulation” published in IEEE Photonics TechnologyLetters in 1998 by A. Djupsjobacka report the usage of a dual-drivemodulator as both a multiplexer and a modulator. Each of the dualmodulator drive signals operates at one half of the optical data rate.However, no method is proposed or successfully demonstrated forpreceding the data for providing the modulator drive signals or forrecovering the original data from the duobinary optical signal bysymbol-by-symbol detection.

There is need for a duobinary optical transmitter using electroniccircuits at low data rates without a requirement to be linear where theoriginal data is recoverable with an optical receiver bysymbol-by-symbol detection.

SUMMARY OF THE INVENTION

The present invention is a method and optical transmitter usingelectronic circuits operating at one-half data rate where the circuitsoperate without a requirement of linearity for generating an opticalsignal having full-rate duobinary modulation and where the original datais recoverable with an optical receiver by symbol-by-symbol detection.

Briefly, a preferred embodiment of an optical transmitter of the presentinvention includes a precoder and a multiplex modulator. The precoderuses two exclusive-OR gates and a one symbol delay component forcalculating two cumulative cross parities for two input data streams.The multiplex modulator includes a one-half symbol delay component,modulation drivers and a dual-drive optical modulator. The one-halfsymbol delay component delays one of the cumulative cross parity streamsby one-half symbol time with respect to the other. The modulationdrivers amplify the cumulative cross parities either before or after theone-half symbol delay for driving the optical modulator. The opticalmodulator modulates an optical signal with a modulation drive signalcorresponding to the difference between the one-half symbol delayedcumulative cross parity stream-stream and the other cumulative crossparity stream for providing a duobinary optical signal having an opticalelectric field having an intensity that may be detected symbol-by-symbolfor recovering the original data in the two input data streams.

An advantage of the present invention is that half-rate precoder andmodulator driver circuits are used for generating full-rate duobinarymodulation on an optical signal from which the original data can besimply detected without decoding. Because the modulator drive signalsare binary, another advantage is that the modulation drivers can beoperated as nonlinear amplifiers.

A duobinary optical signal has three states—a low (zero) field state, apositive field state having a phase angle of 0 radians, and a negativefield state having a phase angle of π radians. This signal is sometimescalled a phase duobinary signal in order to distinguish it from anamplitude duobinary signal having three amplitudes all at the samephase. A rapid transition between phase states of an optical signal maycause frequency chirp. Frequency chirp is undesirable because it spreadsthe frequency band of signal energy. However, a conventional phaseduobinary optical system avoids this frequency chirp by using a balancedmodulator drive signal composed of two simultaneous signals for drivinga dual-drive modulator. The dual-drive modulator uses the simultaneoussignals for modulating two portions of an optical carrier simultaneouslyin equal and opposite directions of phase rotation and combines the twoportions for providing the duobinary optical signal. Alternatively, asingle drive balanced Mach-Zehnder modulator can internally split asingle drive signal input between two waveguide arms. Each of thewaveguide arms modulates a portion of the optical signal. The effect ofthe equal and opposite phase rotation is to cancel the optical signalduring the transitions between phase states so that there is little orno phase change during the transition except when there is zerointensity at the instant in time when the duobinary optical signal flipsbetween phase states. Because there is little no phase change exceptwhen there is zero intensity, there is little or no energy spread by thefrequency chirp in a conventional duobinary system.

The present invention of a phase duobinary optical system also uses amodulator drive signal composed of two signals for driving a dual-drivemodulator. However, in the present invention the two signals may occurone at a time. The dual-drive modulator uses the two signalsindependently for modulating two portions of an optical carrier formaking independent transitions from one phase state to another andcombines the two portions for generating the duobinary output opticalsignal. Because the drive signals may occur one at a time, the presentinvention does not avoid frequency chirp by canceling the optical signalin the conventional manner with equal and opposite phase rotationsduring the state transitions.

In order to prevent frequency chirp spreading for the present invention,a preferred embodiment of a multiplex modulator includes areturn-to-zero (RZ) modulator. The RZ modulator uses a half rate clockdrive signal for providing an RZ light signal to the dual-drivemodulator or single drive balanced modulator. The dual drive modulatormodulates the RZ light signal with modulation drive signal, as describedabove, corresponding to the difference between the one-half symboldelayed cumulative cross parity stream-stream and the other cumulativecross parity stream for providing an RZ duobinary optical signal havingan optical electric field having an intensity that may be detectedsymbol-by-symbol for recovering the original data in the two input datastreams. The clock drive signal is timed so that the RZ duobinary outputoptical signal has full intensity during mid-symbol times and little orno intensity during the state transitions, thereby minimizing thespreading effect of the frequency chirp.

Therefore, an advantage of the present invention is that half-rate dataprocessing and nonlinear modulator drivers are used for generating afull rate data duobinary optical signal from which the original data canbe simply detected without decoding, while at the same time frequencychirp is avoided.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the best mode which isillustrated in the various figures.

IN THE DRAWINGS

FIG. 1 is a block diagram showing a duobinary transmitter of the presentinvention using half-rate signal processing for providing a full-rateduobinary optical signal;

FIG. 1A is a block diagram showing a return-to-zero embodiment of aduobinary transmitter of the present invention using half-rate signalprocessing for providing a full-rate RZ duobinary output optical signal;

FIG. 1B is a block diagram of showing an amplitude modulator forproviding an RZ light signal for the duobinary transmitter of FIG. 1A;

FIGS. 2A-B are time charts of first and second exemplary half-rate inputdata streams to the duobinary transmitters of FIGS. 1 and 1A;

FIGS. 2C-D are time charts of first and second cumulative cross paritystreams in the duobinary transmitters of FIGS. 1 and 1A for the inputdata streams of FIGS. 2A-B;

FIGS. 2E-F are time charts of first and second modulator drive signalsin the duobinary transmitters of FIGS. 1 and 1A for the input datastreams of FIGS. 2A-B;

FIG. 2G is a time chart of a duobinary optical electric field providedby the duobinary transmitter of FIG. 1 for the input data streams ofFIGS. 2A-B;

FIG. 2H is a time chart of an intensity of the duobinary opticalelectric field of FIG. 2G;

FIG. 3A is a transfer characteristic for the optical electric field of adual-drive modulator of the duobinary transmitters of FIGS. 1 and 1A;

FIG. 3B is a transfer characteristic for the intensity of the opticalelectric field of a dual-drive modulator of the duobinary transmittersof FIGS. 1 and 1A;

FIG. 4 illustrates an experimental setup for verifying the multiplexingand modulating functions of the duobinary transmitters of FIGS. 1 and1A;

FIG. 5 illustrates measured waveforms for the experimental setup of FIG.4;

FIG. 6A is a time chart of a full rate clock signal of the duobinarytransmitter of FIG. 1A;

FIG. 6B is a time chart of an RZ light signal of the duobinarytransmitter of FIG. 1A for the clock signal of FIG. 6A;

FIG. 6C is a time chart of an RZ duobinary optical electric fieldprovided by the duobinary transmitter of FIG. 1A for the RZ opticalsignal of FIG. 6B and the input data streams of FIGS. 2A-B;

FIG. 6D is a time chart of an intensity of the duobinary opticalelectric field of FIG. 6C; and

FIG. 6E is a transfer function of the amplitude modulator of FIG. 1B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a duobinary transmitter 10 of thepresent invention having a precoder 11 and a multiplex modulator 12. Theprecoder 11 uses first and second exclusive-OR gates 20 and 21,respectively, and a one-symbol delay component 22 for receiving firstand second binary half-rate input data streams D₁ (t), denoted by 25 a,and D₂(t), denoted by 25 b, respectively, and computing first and secondcumulative cross parity streams P₁(t), denoted by 26 a, and P₂(t),denoted by 26 b, respectively. The first and second data streams D₁(t)25 a and D₂(t) 25 b taken together carry the full-rate data that is tobe transmitted.

The first exclusive-OR gate 20 provides the first cumulative crossparity stream 26 a P₁(t) equal to P₂(t−T)+D₁(t)mod 2, and the secondexclusive-OR gate 21 provides the second cumulative cross parity stream26 b P₂(t) equal to P₁(t)+D₂(t)mod 2, where the T is a half-rate inputsymbol time corresponding to the symbols in the half-rate inputdata-streams D₁(t) 25 a and D₂(t) 25 b. Recursive operation of the firstexclusive-OR gate 20 results in the first cumulative cross parity stream26 a P₁(t) of D₁(t)+D₂(t−D)+D₁(t−T)+D₂(t−2T)+D₁(t−2T)+D₂(t−3T)+D₁(t−3T)+. . . modulo 2 as the cumulative cross parity of the first data streamD₁(t) 25 a and the second data stream 25 b one symbol delayed D₂(t−T).It should be noted that the first cumulative cross parity stream 26 aP₁(t) is the cross parity of the first data stream D₁(t) 25 a and theone symbol delayed second data stream D₂(t−T) plus the previous firstcumulative cross parity.

Similarly, recursive operation of the second exclusive-OR gate 21results in the second cumulative cross parity stream 26 b P₂(t) ofD₂(t)+D₁(t)+D₂(t−T)+D₁(t−T)+D₂(t−2T)+D₁(t−2T)+D₂(t−3T)+D₁(t−3T)+ . . .modulo 2 as the cumulative cross parity of the second data stream D₂(t)25 b and the first data stream 25 a. It should be noted that the secondcumulative cross parity stream 26 b P₂(t) is the cross parity of thesecond data stream D₂(t) 25 b and the first data stream D₁(t) plus theprevious second cumulative cross parity. Filters may be inserted forfiltering the cumulative cross parity streams P₁(t) 26 a and P₂(t) 26 bbetween the precoder 11 and the multiplex modulator 12.

The multiplex modulator 12 includes a dual-drive Mach-Zehnder opticalmodulator 27 and a light source 28. The light source 28 provides inputlight 29 to the optical modulator 27. The optical modulator 27 modulatesthe input light 29 with first and second modulator drive signals V₁(t),denoted by 40 a, and V₂(t), denoted by 40 b, respectively. A firstmodulator driver 41 a amplifies the first precoder output (firstcumulative cross parity stream) 26 a for providing the first modulatordrive signal 40 a. A second modulator driver 41 b amplifies the secondprecoder output (second cumulative cross parity stream) 26 b before orafter the second precoder output 26 b is delayed by a one-half symboldelay component 42 by half the input symbol time (T/2). Because themodulator drive signals V₁(t) 40 a and V₂(t) 40 b are binary, themodulator drivers 41 a and 41 b may be limiting, saturated, or nonlinearamplifiers without a linearity requirement. The optical modulator 27 isbiased with a bias voltage V_(b), denoted by 45, for providing amodulator output signal 50. The bias voltage V_(b) 45 is set so that themodulator output signal 50 is minimized when the voltages of the firstand second modulator drive signals 40 a and 40 b are equal.

FIG. 1A is a block diagram of a duobinary transmitter of the presentinvention having a reference number 10A. The duobinary transmitter 10Aincludes the precoder 11 described above and a multiplex modulator 12A.The multiplex modulator 12A includes the dual-drive Mach-Zehnder opticalmodulator 27, the light source 28, the first modulator driver 41 a, thesecond modulator driver 41 b, and the one-half symbol delay component 42described above. The multiplex modulator 12A also includes areturn-to-zero (RZ) amplitude modulator 30. The modulator 30 amplitudemodulates the input light 29 with a clock signal C(t) (FIG. 6A) inon-off cycle for providing an RZ light signal having a pulsed intensityI_(NZ)(t) (FIG. 6B). The functions of the light source 28 and the RZmodulator 30 may be combined in an RZ light source 31. The opticalmodulator 27 modulates the RZ light signal with the modulator drivesignals V₁(t) 40 a and V₂(t) 40 b for providing a return-to-zero (RZ)duobinary output signal 50A having an optical electric field E_(O)(t)(FIG. 6C) and an output optical intensity I_(O)(t) (FIG. 6D).

FIG. 1B shows the RZ modulator 30 as a zero-chirp balanced Mach-Zehnderdevice having a drive range V_(π). The modulator 30 can be constructedas a single drive balanced Mach-Zehnder Interferometer (MZI) device withan X-cut using LiNbO3 for receiving the clock signal C(t) at a singleinput. Alternatively, the MZI device can be constructed with a dualinput drive for receiving the clock signal C(t) at a both inputs. Theclock signal C(t) operates at the rate of the first and second half ratedata streams D₁(t) and D(t) and is synchronized with the data streamsD₁(t) and D₁(t). The peak-to-peak amplitude of the clock signal C(t) isnominally twice the range V_(π) of the RZ modulator 30. The RZMach-Zehnder modulator 30 is biased with a voltage V_(bNZ) for providingpulses for the intensity I_(NZ)(t) at twice the rate of the clock signalC(t) according to a transfer function shown in FIG. 6E. In anotherimplementation, a clock signal C₂(t) operates a twice the rate of thefirst and second half rate data streams D₁(t) and D₁(t) (twice the rateof the clock signal C(t) shown in FIG. 6A) and is synchronized with thedata streams D₁(t) and D₁(t). In this implementation, the peak-to-peakamplitude of the clock signal C₂(t) is nominally the same as the rangeV_(π) of the RZ modulator 30 and the modulator 30 is biased at a voltageV_(bNZ)+V_(π)/2 or a voltage V_(bNZ)−V_(π)/2 for providing pulses forthe intensity I_(NZ)(t) at the same rate as the clock signal C₂(t)according to the transfer function shown in FIG. 6E.

FIGS. 2A and 2B show exemplary first and second binary input datastreams 25 a D₁(t) and 25 b D₂(t), respectively, versus time t. The timet is shown in units of the half-rate input symbol time T. FIGS. 2C and2D show the first and second cumulative cross parities streams (firstand second precoder output symbol streams) 26 a P₁(t) and 26 b P₂(t),respectively, responsive to the exemplary input data streams 25 a and 25b, versus the time t. FIGS. 2E and 2F show the first and secondmodulator drive signals V₁(t) 40 a and V₂(t) 40 b, respectively,responsive to the exemplary input data streams 25 a and 25 b, versus thetime t. The modulator drive signals 40 a and 40 b have a timing offsetof T/2 (one-half the half-rate input symbol time), versus the time t.

FIGS. 2G and 2H show the optical signal 50 (FIG. 1) in the form of anoptical electric field E(t), denoted by 50 a, and an optical intensityI(t), denoted by 50 b, respectively, responsive to the exemplary inputdata streams 25 a and 25 b, versus the time t. Note that the beginningtime t from 0 to T/2 of the signals 40 b, 50 a, and 50 b cannot bederived from the input data streams 25 a and 25 b. Importantly, itshould be noted that the optical intensity I(t) 50 b corresponds to themultiplexed data in the combination of the first and second data inputdata streams D₁(t) 25 a and D₂(t) 25 b, thereby enablingsymbol-by-symbol recovery by an intensity detector of the full-rateoriginal data.

FIG. 3A shows an electric field transfer characteristicE_(out)(t)/E_(in)(t), denoted by 60 a, of the optical modulator 27 withrespect to the difference V₁(t)−V₂(t) between first and second modulatordrive signals V₁(t) 40 a and V₂(t) 40 b. FIG. 3B shows an intensitytransfer characteristic I_(out)(t)/I_(in)(t) denoted by 60 b, of theoptical modulator 27 with respect to the difference V₁(t)−V₂(t) betweenfirst and second modulator drive signals V₁(t) 40 a and V₂(t) 40 b. Thepeak-to-peak signal swing for each of the modulator drive signals 40 aand 40 b is equal to the maximum peak input V_(π) specified for themodulator 27.

Using the transfer characteristic 60 a in FIG. 3A in terms of an opticalelectrical field, the modulator output signal 50 has the opticalelectrical field of 50 a that is shown in FIG. 2G. The opticalelectrical field of 50 a is a duobinary signal with the followingproperties: a) the signal has the same sign if there are even number ofzeros in between; b) the signal changes sign if there are odd number ofzeros in between; c) there is no direct transition from positive tonegative electrical field and vice versus without first through the zerostate.

Using the transfer characteristic 60 b in FIG. 3B in term of intensity,the modulator output signal 50 has the intensity of 50 b that is shownin FIG. 2H. Comparing the waveform of the output intensity 50 b with theinput data streams 25 a and 25 b, it is seen that the intensity 50 b isa multiplexed signal of both 25 a and 25 b. If a photodetector is usedto detect the intensity of 50 b, no decoder but a demultiplexer isrequired to recover the original data in the waveforms of 25 a and 25 b.Importantly, the intensity waveform of 50 b has twice the data-rate ofthe input data streams of 25 a and 25 b.

FIG. 4 illustrates an experimental setup for demonstrating the functionof the multiplexing modulator 12 using two 10 Gb/s pattern generators100 a and 100 b to give two independent 23³¹−1pseudo-random-bit-sequences (PRBS). The output 50 of the multiplexingmodulator 12 is passed to a photodetector 101 followed by anoscilloscope 102.

FIG. 5 shows measured eye-patterns 110 a, 110 b and 111, respectively,at the oscilloscope 102, when the bias voltage V_(b) 45 (FIG. 1) isproperly adjusted for each individual case. The eye-pattern 110 a isrecorded when the pattern generator 110 a is operating and the patterngenerator 100 b is not operating. The eye-pattern 110 b is recorded whenthe pattern generator 100 b is operating and the pattern generator 100 ais not operating. The eye-pattern 111 is recorded when both the patterngenerator 110 a and 100 b are operating. Comparing eye-patterns 110 a,110 b and 111 confirms the operation of the multiplex modulator 12.

FIG. 6A shows the clock signal C(t). FIG. 6B shows the RZ light signal11 z(t). FIG. 6C shows the optical electric field signal E_(O)(t) forthe RZ duobinary output signal 50A. The optical electric field signalE_(O)(t) carries information symbols in three states—a low (zero) levelfield 52 e, a high (non-zero) field of a positive polarity 53 e, and ahigh (non-zero) field of a negative polarity 54 e. The timing of theclock signal C(t) is controlled so that the optical electric fieldE_(O)(t) has an intersymbol low (zero) level field 51 e during phasetransitions between the symbols. FIG. 6D shows the intensity I_(O)(t) ofthe RZ duobinary output signal 50A. The intensity I_(O)(t) has a low(zero) level intensity 51 i (FIG. 6D) corresponding in time to the lowlevel field 51 e between the informational states, a low level intensity52 i (FIG. 6D) corresponding in time to the low (zero) field 52 e, ahigh level intensity 53 i (FIG. 6D) corresponding in time to the highlevel positive field 53 e, or a high level intensity 54 i (FIG. 6D)corresponding in time to the high level negative field 54 e.

FIGS. 6C and 6D show the optical symbol states carrying information inthe optical signal 50A in the form of an optical electric field E_(O)(t)and an optical intensity I_(O)(t) responsive to the exemplary input datastreams 25 a and 25 b, versus the time t. Note that the beginning time tfrom 0 to T/2 of the signals 40 b and 50A cannot be derived from theinput data streams 25 a and 25 b. Importantly, it should be noted thatthe optical intensity I_(O)(t) corresponds to the multiplexed data inthe combination of the first and second data input data streams D₁(t) 25a and D₂(t) 25 b, thereby enabling symbol-by-symbol recovery by anintensity detector of the full-rate original data.

FIG. 6E is a transfer function 55 of the RZ modulator 30. The transferfunction 55 shows intensity output I_(output) versus the drive voltageinput V_(input) of the modulator 30. The V_(bNZ) bias and the amplitudeof the clock signal C(t) are set so that the modulator 30 generatespulses at twice the frequency of the clock signal C(t).

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the presentinvention.

1. An optical transmitter, comprising: a precoder for precoding twohalf-rate data streams having half-rate input symbol times into twocumulative cross parity streams, said half-rate data streams carryingdata equivalent to a single full-rate data stream; and a multiplexmodulator for using said parity streams for providing a return-to-zero(RZ) optical signal carrying duobinary RZ modulation having intensitycorresponding to said full-rate data stream; and wherein: a currentparity of a first of said parity streams is a cross parity of a currentsymbol of a first of said half-rate data streams with a delayed secondhalf-rate data stream, plus a last previous parity of said first paritystream, where a second of said half-rate data streams is delayed by saidinput symbol time for providing said delayed second half-rate datastream; and a current parity of a second of said parity streams is across parity of a current symbol of said second half-rate data streamwith a current symbol of said first half-rate data stream, plus a lastprevious parity of said second parity stream.
 2. The transmitter ofclaim 1, wherein: said duobinary modulation corresponds to a differencebetween a half-delayed second parity stream and said first parity streamwhere said second parity stream is delayed by one-half said input symboltime with respect to said first parity stream for providing saidhalf-delayed second parity stream.
 3. An optical transmitter,comprising: a precoder for precoding two half-rate data streams havinghalf-rate input symbol times into two cumulative cross parity streams,said half-rate data streams carrying data equivalent to a singlefull-rate data stream; and a multiplex modulator for using said paritystreams for providing a return-to-zero (RZ) optical signal carryingduobinary RZ modulation having intensity corresponding to said full-ratedata stream; and wherein: the precoder includes a symbol delay componentfor delaying a second of said parity streams by said input symbol timefor providing a delayed parity stream; a first gate for exclusive-ORcombining a first of said half-rate data streams and said delayed paritystream for providing a first of said parity streams; and a second gatefor exclusive-OR combining a second of said half-rate data streams withsaid first parity stream for providing said second parity stream.
 4. Thetransmitter of claim 3, wherein: the multiplex modulator includes aone-half symbol delay component for delaying said second parity streamby one-half said input symbol time with respect to said first paritystream for providing a half-delayed second parity stream, and an opticalmodulator for modulating an RZ light signal with a modulation drivecorresponding to a difference between said half-delayed second paritystream and said first parity stream for providing said duobinarymodulation as an optical electric field having said intensity.
 5. Thetransmitter of claim 4, wherein: the multiplex modulator includes an RZlight source for providing said RZ light signal, a dual-drive opticalmodulator for multiplexing said half-delayed second parity stream andsaid first parity stream onto said RZ light signal for forming saidoptical electric field.
 6. An optical transmitter, comprising: aprecoder for precoding two half-rate data streams having half-rate inputsymbol times into two cumulative cross parity streams, said half-ratedata streams carrying data equivalent to a single full-rate data stream;and a multiplex modulator for using said parity streams for providing areturn-to-zero (RZ) optical signal carrying duobinary RZ modulationhaving intensity corresponding to said full-rate data stream; andwherein: said RZ optical signal has an optical electric field havingintersymbol low field levels and field states including low field statesand non-zero field states, said intersymbol low field levels interleavedbetween said field states, said intensity greater in said non-zero fieldstates than in said intersymbol low field levels and said low fieldstates; and said optical electric field has one of said non-zero fieldstates having a first field sense when there are an even number of saidlow field states following a last one of said non-zero field states; hasa change in said field sense when there are an odd number of said lowfield states following said last non-zero field state; and has no changein said field sense until at least one of said low field states aftersaid last non-zero field state.
 7. A method for transmitting an opticalsignal, comprising: precoding two half-rate data streams havinghalf-rate input symbol times into two cumulative cross parity streams,said half-rate data streams carrying data equivalent to a singlefull-rate data stream; and multiplexing representations of said paritystreams onto a return-to-zero (RZ) optical signal as duobinary RZmodulation having intensity corresponding to said full-rate data stream;and wherein: a current parity of a first of said parity streams is across parity of a current symbol of a first of said half-rate datastreams with a delayed second half-rate data stream, plus a lastprevious parity of said first parity stream, where a second of saidhalf-rate data streams is delayed by said input symbol time forproviding said delayed second half-rate data stream; and a currentparity of a second of said parity streams is a cross parity of a currentsymbol of said second half-rate data stream with a current symbol ofsaid first half-rate data stream, plus a last previous parity of saidsecond parity stream.
 8. The method of claim 7, wherein: said duobinarymodulation corresponds to a difference between a half-delayed secondparity stream and said first parity stream where said second paritystream is delayed by one-half said input symbol time with respect tosaid first parity stream for providing said half-delayed second paritystream.
 9. A method for transmitting an optical signal, comprising:precoding two half-rate data streams having half-rate input symbol timesinto two cumulative cross parity streams, said half-rate data streamscarrying data equivalent to a single full-rate data stream; andmultiplexing representations of said parity streams onto areturn-to-zero (RZ) optical signal as duobinary RZ modulation havingintensity corresponding to said full-rate data stream; and wherein: thestep of precoding includes delaying a second of said parity streams bysaid input symbol time for providing a delayed parity stream;exclusive-OR combining a first of said half-rate data streams and saiddelayed parity stream for providing a first of said parity streams; andexclusive-OR combining a second of said half-rate data streams with saidfirst parity stream for providing said second parity stream.
 10. Themethod of claim 9, wherein: the step of multiplexing includes delayingsaid second parity stream by one-half said input symbol time withrespect to said first parity stream for providing a half-delayed secondparity stream; and modulating an RZ light signal with a modulation drivecorresponding to a difference between said half-delayed second paritystream and said first parity stream for providing said duobinary RZmodulation as an optical electric field having said intensity.
 11. Themethod of claim 10, wherein: the step of multiplexing includesgenerating an RZ light signal and using a dual-drive optical modulatorfor multiplexing said half-delayed second parity stream and said firstparity stream onto said RZ optical light signal for forming said opticalelectric field.
 12. A method for transmitting an optical signal,comprising: precoding two half-rate data streams having half-rate inputsymbol times into two cumulative cross parity streams, said half-ratedata streams carrying data equivalent to a single full-rate data stream;and multiplexing representations of said parity streams onto areturn-to-zero (RZ) optical signal as duobinary RZ modulation havingintensity corresponding to said full-rate data stream; and wherein: saidRZ optical signal has an optical electric field having intersymbol lowfield levels and field states including low field states and non-zerofield states, said intersymbol low field levels interleaved between saidfield states, said intensity of said non-zero field states greater thansaid intersymbol low field levels or said low field states; and the stepof multiplexing includes modulating said optical signal for said opticalelectric field having one of said non-zero field states having a firstfield sense when there are an even number of said low field statesfollowing a last one of said non-zero field states; having a change insaid field sense when there are an odd number of said low field statesfollowing said last non-zero field state; and having no change in saidfield sense until at least one of said low field states after said lastnon-zero field state.